Strange Shadow of Invisibility

Hey again all.
I'm having a weird issue with Nippon. There seems to be a single shadow hovering over the entire map at a fixed height. One moment, everything will be lit properly, the next, there's this large shadow.
Anything above where the shadow begins is lit perfectly fine. Anything below it will be lit perfectly fine by lights (such as spotlights and whatnot) but not by the sky.
I don't get it. I've added a 3D skybox with some skycards, but I just checked some decompiled Valve maps and those don't have their shadows disabled.
There's nothing in the .vmf that is fixed at that height, no hint brush or anything. It seems to be a perfectly arbitrary height, and everything below it is covered in shadow.
The map has no leaks; I haven't changed the sky lighting or anything at all since the last build. The only real change besides the skybox was adding in some extra brushwork and displacements for a RED respawn room, but that doesn't seem to make any sense, either.
I do get the "Displacement edge abutting multiple other edges" error, but I've had that for the longest time without issues. I've also had that degenerate face error in there for a while without any issues until now.

This is what I mean:
Notice that the shadow is a just a single line across the entire BSP; anything below it (outside) is covered in shadow. All interior lighting is lit properly by the interior lights.

This last one was accidental, but I forgot to take any real screenshots of the inside, so this'll have to prove that the lighting inside is fine:

This seems to be such a weird and arbitrary error. I don't get it. Any help?

Interlopers says that: "This is no real error, but it could be the cause of lighting anomalies. There is no easy way to find the displacement either, the fastest way would be to go through your displacements one by one."

Interlopers says that: "This is no real error, but it could be the cause of lighting anomalies. There is no easy way to find the displacement either, the fastest way would be to go through your displacements one by one."

Click to expand...

Yeah, I've had that error forever. I don't think it's exactly the cause of it; the lighting anomalies it speaks of are more along the lines of one displacement completely black. This affects regular world geometry as well, so I don't think it's it.

Alright, disabled shadows on all the props in my 3D Skybox. Still have the issue.
I'm going to try to use the nice lightmaps as seen in this tutorial. Maybe it'll help.
E: Negative. Fancy lighting just made it a lot darker. :/
EE: I went ahead and hid the props, did a compile, no dice. Did the same with displacements and with brush entities. Also no dice. I don't get it.